This relates generally to imaging systems and, more particularly, to charge-coupled device (CCD) imaging systems.
Electronic devices such as cellular telephones, cameras, and computers often include imaging systems that include digital image sensors for capturing images. Image sensors may be formed having a two-dimensional array of image pixels that contain photodiodes that convert incident photons (light) into electrical signals. Electronic devices often include displays for displaying captured image data.
Conventional interline CCD imagers are provided with rows and columns of photodiodes. Each column of photodiodes generate charge that is transferred to a corresponding vertical CCD (VCCD). The charge is then read out from each VCCD to a horizontal CCD (HCCD), where the charge can then fed to an associated output circuit. The output circuit typically includes one or more load transistors in its current sink path. In this configuration, the load transistor in the current sink path will often experience a high voltage drop of up to 10 volts or more across its source-drain terminals. A conventional load transistor with self-aligned n+ source and drains may have a breakdown voltage that is substantially less than the 10 V voltage drop due to high electric fields at the drain, making that transistor unusable in the output circuit. High electric fields at the-drain terminals can also result in light being emitted from the current load transistor, which is manifested as an undesired glow in the final image in the region near the output circuit.
It is within this context that the embodiments herein arise.